1. Field of the Invention
The present invention relates to an image forming optical system which can be used for an imaging unit with the solid-state imaging element of CCD, CMOS and the like. For example, it relates to an image forming optical system which can be used for a miniature camera and a monitor camera and the like which are equipped in, for example, a digital still camera, a digital video camera, a cellular phone, PC and the like.
The present invention also relates to an electronic instrument such as a digital still camera, a digital video camera, a cellular phone, PC and the like which use the image forming optical system.
2. Description of the Related Art
In recent years, electronic cameras for taking a photograph by using a solid-state imaging element like CCD and CMOS instead of using silver salt film have become popular.
In such electronic cameras, for an imaging unit which is equipped in a portable type computer or a cellular phone and the like, miniaturization and weight-lightening have been particularly demanded.
As an image forming optical system used for such imaging unit, conventionally there are systems constituted with single lens or two lenses. However, it has been already known that in the image forming optical system having such constitution as mentioned above, curvature of field cannot be corrected and high performance cannot be expected. Therefore, it is necessary to constitute with three or more lenses are required in order to achieve high performance in an image forming optical system.
On the other hand, when an imaging element like CCD is used for an imaging unit, if off-axis luminous flux emanated from an image forming optical system enters with too large incident angle to an image plane, condensing performance of a micro lens is not fully exercised. Accordingly, problem that the brightness of a picture image changes extremely in the central portion and the circumferential portion of the picture image arises.
Therefore, the incident angles of the light to an imaging element as CCD or the like, that is, the position of an exit pupil becomes important in designing. Furthermore, the position of an aperture stop becomes important when an optical system is constituted with small number of lenses.
As systems in which these problems are taken into consideration, there are optical systems where an aperture stop is arranged from an object side before the first lens, or between the first lens and the second lens. As conventional examples, such image forming optical systems have been disclosed in the following documents such as Japanese published unexamined patent applications: Toku Kai Hei 05-188284, Toku Kai Hei 07-27974, Toku Kai Hei 09-288235, Toku Kai Hei 11-52227 and Toku Kai 2001-83409.